add time zone to recommended iso format

[crotwell]

My understanding of the ISO spec is that if there is no time zone, it is assumed to be in local time. Since seismology uses mostly UTC, I think it would be good to require the 'Z' to indicate UTC on the times. That way it is the time that specifies the time zone and so general purpose parsers will do the right thing.
So make it:
Date and time in one form of ISO 8601, specifically YYYY-MM-DDThh:mm:ss[.SSSSSS]Z

[CTrabant]

The 'Z' is already required, none of the examples would validate without it. I will update the description of the time format to:
a) clarify this point, always UTC and preferably with the Z suffix (alternatively with +00)
b) refer to RFC3339, which is what JSON Schema requires.